1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to imaging lenses which are built in image pickup devices mounted in increasingly compact and low-profile mobile terminals such as smart phones, mobile phones and PDAs (Personal Digital Assistants), and game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of mobile terminals including smart phones has been ever-expanding, and in the field of cameras built in mobile terminals, products which cope with an increase in the number of pixels have become mainstream. Imaging lenses for such cameras are strongly expected to provide higher resolution and be more compact and low-profile and also to realize a lens system which ensures high brightness to cope with an increase in the number of pixels and provides a wide field of view to capture an image of an object over a wide range.
Many types of imaging lenses composed of four constituent lenses (four-element lenses) which are relatively compact and provide high performance have been proposed as imaging lenses which meet the trend toward higher performance as mentioned above.
For example, JP-A-2008-033327 (Patent Document 1) discloses an imaging lens which includes a first lens with positive refractive power as a biconvex lens, a second lens with negative refractive power as a meniscus lens having a convex surface on an object side, a third lens with positive refractive power as a meniscus lens having a convex surface on an image side, and a fourth lens with negative refractive power, in which the length of the overall optical system in the optical axis direction and the ratio of the focal length of the first lens to the overall focal length are set within appropriate ranges in order to achieve compactness and high performance.
JP-A-2009-265245 (Patent Document 2) discloses an imaging lens which includes, in order from an object side, a first lens with positive refractive power having a convex surface on the object side, an aperture stop for adjusting the quantity of light, a second lens with negative refractive power, a third lens with positive refractive power having a concave surface on the object side, and a fourth lens with negative refractive power, in which the ratio between the focal length of the overall optical system and the focal length of the second lens and the ratio between the focal length of the second lens and the curvature radius of the object-side surface of the second lens are set within appropriate ranges in order to achieve compactness and high performance.
The imaging lens described in Patent Document 1 is relatively compact in which TLA/2ih is about 1.0 where TLA denotes total track length and 2ih denotes the diagonal length of the effective imaging plane of the image sensor. However, its F-value is about 3.0, so its brightness is not considered to be sufficient for an image sensor which deals with an increasing number of pixels. Also, its half field of view is in the range of 31 to 32 degrees, which is insufficient to meet the demand for a wide field of view.
The imaging lens described in Patent Document 2 is also relatively compact in which TLA/2ih is about 1.0. However, its F-value is about 2.8, so its brightness is not considered to be sufficient for an image sensor which deals with an increasing number of pixels. Also, its half field of view is in the range of 31 to 32 degrees, which is insufficient to meet the demand for a wide field of view.
Therefore, in the conventional technology, it is difficult to meet the demands for compactness, a wider field of view and a small F-value at the same time.